Re: Data corruption on i.MX6 IPU in arm_copy_from_user()

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Wed, May 26, 2021 at 10:26:50AM +0200, Krzysztof Hałasa wrote:
> Hello,
> 
> I've encountered an interesting case of data corruption while accessing
> IPU (Image Processing Unit) on i.MX6 (rev1.2, Cortex A9). What I'm doing
> here is basically:
> 
> openat(AT_FDCWD, "/dev/mem", O_RDWR|O_SYNC) = 3
> mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, 3, 0x2630000) = ptr
> write(1, ptr, 32)                = 32
> 
> Normally, the write() should end up with:
>  04008A00 02FF03FF 02FF03FF 00000000 00000000 00000000 00000000 00000000
> 
> However, with current kernels, the first 32 bits (the first IPU
>  register) are dropped:
>  02FF03FF 02FF03FF 00000000 00000000 00000000 00000000 00000000 00000000
> 
> 0x2630000 is IPU1 CSI0 address (i.e., a register block). The same
> happens with other IPU regions. Writes shorter than 8 * 32 bits are not
> affected.
> 
> write() uses arm_copy_from_user() and since commit f441882a5229:
>     ARM: 8812/1: Optimise copy_{from/to}_user for !CPU_USE_DOMAINS
> 
>     ARMv6+ processors do not use CONFIG_CPU_USE_DOMAINS and use privileged
>     ldr/str instructions in copy_{from/to}_user.  They are currently
>     unnecessarily using single ldr/str instructions and can use ldm/stm
>     instructions instead like memcpy does (but with appropriate fixup
>     tables).
> 
> apparently uses 8 * 32-bit ldmia instruction to copy data:
>     .macro ldr8w ptr reg1 reg2 reg3 reg4 reg5 reg6 reg7 reg8 abort
>     USERL(\abort, ldmia \ptr!, {\reg1, \reg2, \reg3, \reg4, \reg5, \reg6, \reg7, \reg8})
>     .endm
> 
> Before this commit it used ldr instruction (single 32-bit value) and the
> problem didn't show up (reverting f441882a5229 on v5.11 fixes it as
> well). The i.MX6 errata doesn't seem to list this problem.
> 
> I wonder what the theory says about this case. Is it at all valid to
> read 8 IPU registers at a time using LDM instruction? If so, should
> something be done with this problem, or should it be left as is?

Surely someone is not using copy_*_user() to copy data from userspace
direct to MMIO space... that would be crazy.

-- 
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